I think the point isn't that Giorgi can't start at the 5, it's that if you're playing 4 guards and a 5, you want that 5 to be big. 4 guards and an undersized 5 does not bode well. Giorgi played great his freshman year, but as I recall, we weren't outrebounding many teams before Kofi got on campus.
3 guards, a tall forward, and an undersized 5 could work fine. If Hawkins is ready to start at the 4 next year, that would be ideal. He's actually taller than Giorgi, and has shown flashes of being a very good rebounder and interior defender.